Extrusion rates are often limited by the surface cracks originating at the die exit. These cracks can be so severe in the case of certain polymers that the phenomenon known as "flow split" occurs. Previous work by the same authors explains the macroscopic mechanisms leading to flow split observed during the extrusion of poly styrene-block-poly (ethylene-co-buylene)-block-polystyrene block copolymers in their microphase separated state. The present paper looks at the effects of extrusion on the structure of these block copolymers at regimes showing the different characteristic defects of the fluids. The structure of extruded samples is examined by small-angle x-ray scattering (SAXS). A comparison between the relaxation times characteristic of the copolymers and the time necessary to prepare the samples shows that quenching of samples the flow of which is stable at the die exit is practically impossible with a standard capillary rheometer. Hence, in situ experiments inside the die are necessary to understand the structure changes occurring during the extrusion process. For this reason, a new capillary rheometer equipped with a beryllium die, transparent to x rays, is developed and preliminary tests are performed. The results validate the design and show the feasibility of on-line experiments. Moreover, an interpretation that relates the macroscopic defects and structure observed with SAXS experiments is proposed. (c) 2006 The Society of Rheology.